The present invention relates to a toroidal continuously variable transmission.
Recently, the use of toroidal continuously variable transmissions as motor vehicle transmission has been highlighted again owing to development of traction oil with a high traction factor.
The toroidal continuously variable transmissions may be divided into two kinds, one being referred to as a full toroidal continuously variable transmission, the other a half toroidal continuously variable transmission.
The full toroidal continuously variable transmissions include input and output discs having facing toric surfaces of a 1/2 circular arc, and a plurality of friction rollers disposed between and in driving contact with the toric surfaces for transmitting torque between the input and output discs. These toric surfaces are called as a full toroidal surface. Owing to the full toroidal surfaces, reactions applied to the friction rollers during the transmission of torque between the input and output discs are cancelled. No thurst bearings are needed for supporting the friction rollers, resulting in a simple and durable structure. However, there is a spin loss (a slip loss) during transmission of torque by the friction rollers, and the input and output discs are subject to a considerable inertia of rotation since their radially outermost portions are thick. Taking these two factors into account, the full toroidal continuously variable transmissions are less promising as motor vehicle transmissions.
The half toroidal continuously variable transmissions include input and output discs having facing toric surfaces of a 1/4 circular arc, and a plurality of friction rollers disposed between and in driving contact with the toric surfaces. These toric surfaces are called as a half toroidal surface. Since they have less thick radially outermost portions, the input and output disc are subject to less inertia of rotation. Besides, the spin loss in the half toroidal continuously variable transmissions is small. Thus, the half toroidal consinuously variable transmissions are promising as motor vehicle transmissions.
JP-A 62-283256 (Japanese patent application non-examined publication) discloses a half toroidal continuously variable transmission including two friction rollers disposed between and in driving contact with half toroidal surfaces of input and output discs. Similar troidal continuously variable transmission is disclosed in U.S. Pat. No. 4,905,529 issued to Applicant of the present application on Mar. 6, 1990. This patent claims priority on Japanese patent application Ser. No. 61-275197 which was published as JP-A 63-130954 on June 3, 1988. The known transmission mentioned above cannot meet growing demand for high power output since the two friction rollers can not bear load. For reducing load beared by each of friction rollers, it is advantageous to increase the number of friction rollers.
U.S. Pat. No. 3,142,189 issued to M. Davis et al. discloses a half toroidal continuously variable transmission including three circumferentially-spaced friction rollers disposed between and in driving contact with facing half toroidal surfaces of co-axial input and output discs.
U.S. Pat. No. 3,430,504 discloses a full toroidal continuously variable transmission including two friction roller units, each unit including three circumferentially-spaced friction rollers disposed between facing full toroidal surfaces of coaxial input and output discs.
Recent study of a full toroidal continuously variable transmission with three friction rollers is reported on page 40 of a publication entitled "Report of Traction Drive Research P-SC62" issued on March, 1980 by the Japan Mechanical Society. A ratio control system of this transmission shown in FIGS. 3-37 on page 4 of the above-mentioned publication is illustrated in FIG. 5 of this specification.
Referring to FIG. 5, the transmission comprises circumferentially-spaced friction rollers 1 rotatably supported by three roller carriers 2 called as hubs. Three Y-shaped swing links 3 called as rockers are rotatably supported on a spider 6 grounded to a housing. A control sleeve 7 is rotatably supported on an input shaft. Each of the Y-shaped links 3 has a leg operatively connected to the control sleeve 7 to form a pivotal connection, and first and second arms connected to adjacent two roller carriers. Each of the roller carriers is associated with two Y-shaped links and has one end connected to a first arm of one of the associated Y-shaped links to form a ball joint 4 and an opposite end connected to a second arm of the other associated Y-shaped link 3 to form a joint 5. A change to a desired ratio is initiated by rotating the control sleeve 7 through a small angle. This rotation causes the Y-shaped swing links 3 to swing, tilting the roller carriers 2 through an angle .theta.. This causes a skew (a side slip) to occur between the rollers 2 and those input and output discs which the rollers are disposed between and in driving contact with, causing the rollers to incline relative to the input and oyutput discs.
The above-mentioned full toroidal continuously variable transmission with three friction rollers has the same problems as pointed out before in connection with the full toroidal continuously variable transmission. Thus, it suffers from the before-mentioned spin loss and inertia of rotation of input and output discs. Besides, it is necessary to machine the joint 5 with a high degree of precision in order to position each of the roller carriers with good accuracy.
An object of the present invention is to provide a toroidal continuously variable transmission wherein roller carriers are positioned with good accuracy without any high degree of precision in machining.
Further object of the present invention is to provide a toroidal continuously variable transmission of the above kind which meets a demand for high power output with a spin loss and inertia of rotation of input and output discs restrained.